Specifics can be found in Grand History of the Realms, or on several threads here. The basics are as follows.
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1384 Year of the Three Streams Blooded - Siamorphe and Tyr quarrel over mortal affairs. Siamorphe leaves the House of Triad for Brightwater Sune's domain. Sune suggests marriage between Tyr and Tymora to set the celestial planes back in balance. Helm chaperones the courtship of Tyr and Tymora. Strange misunderstandings lead to the accusation that Helm's stolen Tymora's heart. Tyr is forced to challenge Helm to combat, before either are able to return to their senses Helm is destroyed. Heartbroken Tymora follows Tyr back to the House of the Triad. Although they have no proof the gods believe Cyric had his hand in all of this.
1385 The Year of the Blue Fire - Cyric aided and abetted by Shar murders Mystra in Dweomerheart. The plane disintegrates killing Savras and sending Azuth and Velsharoon hurling deep into the endless Astral Plane. Without Mystra to govern the Weave magic goes wild and destructive. This event is known as the Spellplague. Thousands upon thousands of mages throughout Faerun are either driven insane or killed by the effects of this. Many planes are shifted or destroyed.
Tyr, Lathander, and Sune confront Cyric and imprison him in his Supreme Throne placing him under house arrest for 1000 years.
Sages in centuries to come mark this as the end of the old world and the terrifying beginning of the new.
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Additionally, the Orc King has a prologue set 100 years later then the current setting. The basics are as follows. Link to the Prologue
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- Many old empires have fallen (e.g. Luskan - page 11). Others are named which are perhaps destroyed but perhaps just not prospering (Thay, Mulhorand, Sembia - page 12).
- New ones have arisen (Netheril, the aboleths, and a "discordant and disastrous joining of two worlds") - page 10. I wonder about the last one of this, but it seems to coincide with Ed Greenwood's comments in the final issue of Dragon (along the lines of "most scholars drop the name Abeir from Abeir-Toril, for reasons which will soon become apparent"). The question is, which two worlds?
- The Kingdom of Many-Arrows has flourished and helped bring stability to the Silver Marches region under Obould and his heirs; orcs (and half-orcs) are more accepted than in 1375 DR. The Cult of Gruumsh is de-emphasised in this Kingdom (e.g. page 5 onwards). I presume the dwarves are still around there, too.
- Something regarding this happens at a place or to a person called "Shallows"; perhaps that's what this novel is about, I don't know (page 3).
- Eilistraee and her followers "rose", but presumably failed to achieve what they hoped (page 11). Perhaps Lolth has exterminated the rest of her pantheon? We will have to see what the rest of the Lady Penitent books say.
- Other Good deities may have been killed ("Where are the benevolent gods?" - page 11).
- Oh, and I suppose it confirms that Drizzt at least survives...
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Rich Baker, over at the WoTC FR boards, gave the following reasons as to why the change happened. Link
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Anyway, why is Mystra dead? OK, several reasons, some in-world, some meta-management...
1. It's an appropriate (if grim) finish to the story of Cyric's betrayal begun in the Time of Troubles series. Time to move on and tell new stories about Cyric.
2. It's an appropriate (if grim) finish to the story of Shar's jealousy and desire to seize the portfolio of magic. Time to move on and tell new stories about Shar.
3. We know magic's changing anyway, and it seems like a stretch to say that Mystra would make that change herself. This way we don't have to explain why she would do such a thing. The answer's logical: She wouldn't, but if she's dead then it's completely out of her hands.
4. Many people--players as well as game designers and authors--perceived that Mystra (and her Chosen) were simply too strong. She's seen as far and away the most powerful deity, and her Chosen are clearly the most powerful heroes. Authors actually have to deliberately account for each of the Chosen when they're telling big stories and explain what they're doing or what they're not doing and why.
5. Some Chosen will still be around post-Spellplague, but we really do want to get them out of the spotlight and get your PCs (on the game side) or less superheroic characters (on the novel side) into the picture.
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Little other then that has been confirmed. We don't know when the 4th Edition FRCG (That's Campaign Guide, not Setting) will be set. We don't know the extent of the damage from the Spellplague. We don't know who survives, other then Elminster and Drizzt. Plenty of discussion threads on the Realms though. It would be better to continue any discussion on one of them.